32 ANIMAL BIOLOGY 



devoid of energy, and, through the radiant energy of sunlight, 

 shifts and recombines their elements in such a way that products 

 rich in potential energy result. Protococcus thus exhibits the 

 prime distinguishing characteristic of green plants — the power 

 to construct its own foodstuffs. 



The key to this power of chemical synthesis by light — photo- 

 synthesis — resides in a complex chemical substance called 

 chlorophyll which consists of two very similar but distinct pig- 

 ments. Chlorophyll is segregated in special cytoplasmic bodies, 

 the plastids, and gives to Protococcus during its active phases 

 and to the foliage of plants in general their characteristic green 

 color. Plastids bearing chlorophyll are known as chloroplasts. 

 The chlorophyll arrests and transforms a small part of the energy 

 of the sunlight which reaches it, in such a way that the protoplasm 

 can employ this energy for food synthesis. 



The first great step in the constructive process is a combination 

 of carbon with hydrogen and oxygen to form a carbohydrate. 

 Protococcus gets these elements from carbon dioxide and water 

 by a process of molecular disruption. We know that when char- 

 coal, for instance, is burned, carbon and oxygen unite to form 

 carbon dioxide, and energy in the form of light and heat is liber- 

 ated. Obviously Protococcus must employ an equal amount of 

 energy in separating the carbon and oxygen of carbon dioxide; 

 that is, in overcoming their chemical affinity. And this kinetic 

 energy which the plant employs is then represented in the chemical 

 potential which exists between the oxidizable carbon and free 

 oxygen — it has become potential energy. Thus the plant in sun- 

 light is continually separating the carbon from the oxygen of carbon 

 dioxide. The oxygen is liberated as free oxygen, while the carbon 

 which has been separated from the oxygen is combined with mole- 

 cules of water to form a carbohydrate — grape sugar (glucose) . 



The conventional equation for this reaction is: 



6C0 2 + 6H 2 = C 6 H 12 6 + 6 2 



(carbon dioxide) (water) (glucose) (free oxygea) 



However, the processes involved are by no means so simple as is 

 implied above. It is probable that a relatively simple compound, 

 such as formaldehyde (CH 2 0), is first produced from carbon 

 dioxide and water, and that molecules of this substance are then 

 united to form glucose. Although there is little conclusive data in 



